A natural Urushi liquid (Urushi) is a water-in-oil emulsion, and is formed from urushiol (a lipid component), a gum (a polysaccharide), a nitrogen-containing substance (a glycoprotein), laccase (an enzyme), and water. There have been many reports concerning the chemistry of Urushi liquid, including the reports of Non-patent Publications 1 to 4.
The water droplet particle size of the raw Urushi emulsion is about 10 micrometers (μm), but the average water droplet particle size of a refined Urushi whose moisture content has been reduced by a ‘Nayashi’ mixing and kneading process and a ‘Kurome’ heating process is about 1 μm. This is made into a coating and allowed to stand in an Urushi room at high humidity, the laccase enzyme contained in the water droplets oxidizes the urushiol, and the formation of urushiol quinone, the formation of dibenzofuran, the formation of a quinone-olefin addition polymer, etc. proceed. Due to a decrease in antioxidant power as a result of these reactions, an automatic oxidation reaction of unsaturated side-chain bonds proceeds, and a dried and cured Urushi coating is obtained (ref. reaction schemes in FIG. 1 and FIG. 2; the automatic side chain oxidation reaction proceeds in reality in an urushiol oligomer state or an urushiol polymer state).
It is well known that an Urushi coating exhibits a high-class image with a sense of being soft and comfortable, well-used, and of richness, and will last for a few thousand years, but since the dispersion particle size of the emulsion is relatively large, the gloss of the coating is reduced. Because of this, gloss is imparted to an Urushi coating by adding a polymerized perilla oil, a rosin-modified polymerized linseed oil, etc., wet-rubbing the surface of a dried coating, or carrying out Roiro-finishing by cylinder polishing.
(Non-patent Publication 1) Kisuke Nagase, Yukio Kamiya, Toru Kimura, Kengo Hozumi, and Tetsuo Miyakoshi, Nikka (Journal of the Chemical Society of Japan), No. 10, 587 (2001)
(Non-patent Publication 2) Kisuke Nagase, Yukio Kamiya, Kengo Hozumi, and Tetsuo Miyakoshi, Nikka (Journal of the Chemical Society of Japan), No. 3, 377 (2002).
(Non-patent Publication 3) Kisuke Nagase, ‘Urushi no Hon’ (Urushi Book) (Kenseisha), published September, Showa 61 (1986).
(Non-patent Publication 4) Edited and written by Tetsuo Miyakoshi, Kisuke Nagase, and Takashi Yoshida, ‘Urushikagaku no Shinpo’ (Progress in Urushi Chemistry), (IPC Ltd.), published May, Heisei 12 (2000).